Dicarbonylchloro(pentabenzylcyclopentadienyl)rutheniuni as racemization catalyst in the dynamic kinetic resolution of secondary alcohols

Article abstract of DOI:10.1002/ejoc.200801248

Dicarbonylchloro(pentabenzylcyclopentadienyl)ruthenium has been prepared and its structure confirmed by X-ray analysis. This complex shows excellent catalytic activity and modest stability against air in racemization reactions of secondary alcohols. In Candida antarctica lipase B (CAL-B) catalyzed dynamic kinetic resolution (DKR) of 1-phenyl- and l-(furan-2-yl)ethanol compounds, the new complex shows improved performance as an alcohol racemization catalyst in comparison with its well-known pentaphenylcyclopentadienyl analogue, hitherto considered as the leading catalyst candidate. Wiley-VCH Verlag GmbH & Co. KGaA.

Full text of DOI:10.1002/ejoc.200801248

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200801248
Dicarbonylchloro(pentabenzylcyclopentadienyl)ruthenium as Racemization
Catalyst in the Dynamic Kinetic Resolution of Secondary Alcohols
Denys Mavrynsky,[a] Mari Päiviö,[b] Katri Lundell,[b] Reijo Sillanpää,[c] Liisa T. Kanerva,*[b]
and Reko Leino*[a]
Keywords: Ruthenium / Dynamic kinetic resolution / Cyclopentadienyl ligands / Enzyme catalysis / Racemization
Dicarbonylchloro(pentabenzylcyclopentadienyl)ruthenium has 2-yl)ethanol compounds, the new complex shows improved
been prepared and its structure confirmed by X-ray analysis.
This complex shows excellent catalytic activity and modest
stability against air in racemization reactions of secondary
alcohols. In Candida antarctica lipase B (CAL-B) catalyzed
dynamic kinetic resolution (DKR) of 1-phenyl- and 1-(furan-
performance as an alcohol racemization catalyst in compari-
son with its well-known pentaphenylcyclopentadienyl ana-
logue, hitherto considered as the leading catalyst candidate.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
solvents and substrate scope. Furthermore, the preparation
of ligand precursors for 1 and 2, even starting from the
commercially available tetraphenylcyclopentadienone, is not
straightforward, involving the use of air-sensitive Grignard
reagents, lithium-based reducing agents and dry solvents.
Introduction
Dynamic kinetic resolution (DKR) of racemic sec-
alcohols by combination of enzymes with transition-metal
catalysts, originally developed by Williams[1] and
Bäckvall,[2] is a useful method for the production of chiral
building blocks.[3] In such processes, in-situ racemization of
the slower reacting enantiomer by a metal catalyst enables
theoretical yields of 100%, not reachable in traditional ki-
netic resolutions (KR), which by theory stop at the maxi-
mum yield of 50%. Various metal complexes are known to
racemize alcohols, the key issues of catalytic activity, sta-
bility and compatibility with enzymes being met, however,
by only a few systems.[4] Hitherto, the most successful cata-
lyst design has been based on half-sandwich ruthenium
complexes, e.g., 1–3 (Figure 1), of which the (pentaphen-
ylcyclopentadienyl)ruthenium complex 1 is often claimed as
the currently best candidate.[5] Both catalyst 1 and its
amino-substituted analogue 2[6] can be successfully utilized
in DKR at ambient temperature, whereas the initially em-
ployed Shvo’s catalyst 3 requires higher temperatures for
generating the active monoruthenium species.[2,7] Neverthe-
less, complexes such as 1 and 2 can be considered as being
far from optimal, in terms of cost and ease of preparation,
air and moisture stability, activity, compatibility with polar
Figure 1. Half-sandwich ruthenium complexes 1–5.
Results and Discussion
We have recently prepared a number of half-sandwich,
indenyl-based Ru complexes containing fused aromatic ring
substituents and initially screened their efficiencies as race-
mization catalysts for (S)-1-phenylethanol.[8] In this series
of complexes, enhanced racemization performance (albeit
clearly inferior to that of 1) was observed with the benzo-
yloxy-substituted catalyst 4a, containing a pendant aro-
matic phenyl ring, when compared with its 1-adamantyl-
substituted analogue 4b (Figure 1). This led us to investi-
gate further the pendant aromatic substituent effects in
[a] Laboratory of Organic Chemistry Åbo Akademi University,
20500 Åbo, Finland
E-mail: reko.leino@abo.fi
[b] Pharmacology, Drug Development and Therapeutics/Labora-
tory of Synthetic Drug Chemistry and Department of Chemis-
try, University of Turku,
20014 Turku, Finland
E-mail: lkanerva@utu.fi
[c] Department of Chemistry, University of Jyväskylä,
40351 Jyväskylä, Finland
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Eur. J. Org. Chem. 2009, 1317–1320
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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L. T. Kanerva, R. Leino et al.
SHORT COMMUNICATION
DKR and to prepare a pentabenzylcyclopentadienyl ana-
logue of 1, namely the dicarbonylchlororuthenium complex
5b (Figure 1), the hydride analogue 5a of which has been
described earlier in the context of alcohol dihydrox-
ylation.[9] In comparison to C5Ph5H, the high-yield prepa-
ration of C5Bn5H is both seemingly simple and cost-ef-
ficient, even on a large scale, consisting of the reaction of
dicyclopentadiene with sodium benzylate in BnOH.[10] The
molecular structure of 5b, determined in the present study
(Figure 2), shows the expected η5-bonding of the ruthenium
atom also found in the closely analogous half-sandwich
pentabenzylcyclopentadienyl complexes C5Bn5M(CO)3OTf
(M = Mo, W)[11] and C5Bn5Co(CO)2,[12] being in all essen-
tial structural features also similar to complex 1.[5]
Scheme 1. DKR of rac-6 and rac-8 with 1 and 5b.
Figure 3. Racemization of (S)-6 by complexes 1 (᭿) and 5b (᭹)
(0.2 mol-%) under argon.
Figure 2. X-ray structure of 5b. Hydrogen atoms are omitted for
clarity. Ellipsoids are drawn at 50% probability level.
Complex 5b was first evaluated as a catalyst for racemi-
zation of (S)-1-phenylethanol [(S)-6] (Scheme 1) and the re-
sults compared with those obtained with 1 under identical
conditions. Under argon with 0.2 mol-% catalyst loading,
both catalysts 5b and 1 show similar initial reaction rates.
However, at 50% ee level, the activity of 1 decays by decom-
position, whereas 5b remains effective (Figure 3). At 1 mol-
% concentration under argon, both 5b and 1 result in com-
plete racemization within minutes, displaying activities too
high to be measured accurately. In air, at 1 mol-% concen-
tration, complex 5b shows a considerably better activity
than catalyst 1 providing, after 1 min, higher conversion
than 1 after 5 min (Figure 4).
Figure 4. Racemization of (S)-6 by complexes 1 (᭿), 5b (᭹) and 5c
(᭡) (1 mol-%) in air.
For further investigating the effect of benzyl substituents
in racemization catalysts, the monobenzyl-substituted ana-
logue of 1, complex 5c, was prepared by reaction of depro-
tonated C5Ph4BnH with [Ru(CO)3Cl2]2. The monobenzyl-
substituted ligand precursor was prepared in a manner sim-
ilar to that utilized earlier for the C5Ph5H analogue.[5] Also
the structure of 5c was confirmed by X-ray analysis (Fig-
ure 5)[13] to have essential features analogous to those of 1[5]
and 5b. The racemization activity of 5c in air is similar to
that of 1 (Figure 4) indicating that, for improved perform-
Figure 5. X-ray structure of 5c. Hydrogen atoms are omitted for
clarity. Ellipsoids are drawn at 50% probability level.
Encouraged by the racemization results obtained with 5b,
ance, the cooperative action of several pendant substituents, we next investigated this catalyst vs. 1 in combination with
as in 5b, would be required for gaining benefits in catalyst Candida antarctica lipase B (Novozym® 435, CAL-B on li-
activity and stability.
pophilic PMA carrier) for the dynamic kinetic resolution of
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© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2009, 1317–1320

Ru Complexes for Dynamic Kinetic Resolution of Secondary Alcohols
rac-6 and rac-8 (Scheme 1). The substrates were selected on tries 2 vs. 4 and 6 vs. 9). Under optimized DKR conditions
the basis that their DKR has been thoroughly studied ear- for rac-6 and rac-8 using 4 mol-% Ru loading, 5b is more
lier by Bäckvall.[5] The conventional kinetic resolution of stable than 1, showing considerably higher activity. In 24 h,
rac-6 and rac-8 in dry toluene in the presence of CAL-B both catalysts finally led to 90–95% formation of the ace-
(3 mgmL–1) and isopropenyl acetate (1.5 equiv.) is fast and tates (R)-7 and (R)-9, respectively. By performing the DKR
highly enantioselective (enantiomer ratio E ϾϾ 200) to af- on a preparative scale with 2.49 mmol of rac-6, 2.11 mmol
ford the enantiopure resolution products (S)-6 and (R)-7, (85%, ee = 99%) of (R)-7 was separated by column
and (S)-8 and (R)-9, respectively, at 50% conversion chromatography, in accordance with the 98% yield in the
(Table 1). In addition to fast enantioselective acylation, (S)- reaction mixture. During the DKR, the corresponding
6 and (S)-8 were totally racemized within 2 min by using 5b ketones are produced from the alcohols 6 and 8 in minimal
(1 mol-% loading) in toluene under argon. Accordingly, rac- amounts (2–4%) only.
6 and rac-8 should be well suited as model substrates for
The present study indicates that argon is essential for
DKR in the present study. The DKR results obtained with successful DKR with both 1 and 5b as racemization cata-
catalysts 5b and 1 are collected in Table 2.
lysts. In air, the yields of enantiopure (R)-7 remain at the
level of conventional kinetic resolution (Entries 3 and 7).
Table 1. KR of rac-6 and rac-8 with isopropenyl acetate (1.5 equiv.)
and CAL-B (3 mgmL–1) in toluene (1 mL) at 23 °C.
Conclusions
Entry Substrate Time Conversion ee[(S)-6/(S)-8] ee[(R)-7/(R)-9]
([])
[h]
[%]
(%)
(%)
We have demonstrated that complex 5b shows high sta-
bility in comparison with the well-known and widely used
Bäckvall catalyst 1, allowing highly effective DKR of the
secondary alcohols rac-6 and rac-8. Further studies on the
substrate scope are currently in progress. The two second-
ary alcohols investigated in the present study were trans-
formed into their corresponding (R)-acetates in ee Ն 97%
at 90–95% yield. The enhanced stability of the pentaben-
zylcyclopentadienyl complex 5b as compared to its penta-
phenylcyclopentadienyl analogue 1 may be related to the
better shielding of the metal center towards hydrolysis, thus
hindering catalyst decomposition. Finally, the ease of prep-
aration of 5b and its promising performance may prove it
as a viable substitute for 1 as a racemization catalyst for
future DKR applications.
1
2
3
4
6 (0.2)
6 (0.5)
8 (0.2)
8 (0.5)
3
4
1
50
50
50
50
Ͼ99
98
98
Ͼ99
Ͼ99
98
1.5
98
99
Table 2. DKR of rac-6 and rac-8 with isopropenyl acetate
(1.5 equiv.) and CAL-B preparation (3 mgmL–1) in toluene (1 mL)
in the presence of 1 and 5b, tBuOK and Na2CO3 at 23 °C. Condi-
tions: 0.5  substrate, 4 mol-% of 1 or 5b and 5 mol-% of tBuOK;
0.2  substrate, 10 mol-% of 1 or 5b and 12.5 mol-% of tBuOK.
Entry Substrate Catalyst
Time
[h]
Yield[d] ee[(R)-7/(R)-9]
[%]
([])
[%]
1
2
3
4
5
6
7
8
9
10
11
12
13
6 (0.2)
6 (0.5)
6 (0.5)
6 (0.5)
6 (0.2)
6 (0.5)
1[a]
1[a]
3/24
3/24
24
55/71
48/84
54
Ͼ99/Ͼ99
Ͼ99/Ͼ99
99
1 (air)[a]
1[b]
3/7/24 51/59/88 Ͼ99/Ͼ99/Ͼ99
3/7/24 79/84/72
56/84
53
4/16/24 71/80/73
5b[a]
Ͼ99/Ͼ99/99
Ͼ99/Ͼ99
Ͼ99
5b[a]
3/24
24
Experimental Section
6 (0.5) 5b (air)[a]
6 (0.2)
6 (0.5)
6 (0.5)
8 (0.5)
8 (0.2)
8 (0.5)
5b[b]
5b[b]
5b[c]
1[b]
98/98/97
General Remarks and Crystallography: All manipulations with air-
and moisture-sensitive compounds were performed under argon by
using standard Schlenk techniques. Solvents were purchased from
standard vendors and dried by standard procedures (THF, toluene:
redistilled from Na/benzophenone ketyl; DCM: redistilled from
calcium hydride). NMR spectra were recorded with a Bruker Av-
ance 600 MHz spectrometer. Unresolved signals are denoted as
“ur”. Chiral GC analysis was performed with a gas chromatograph
equipped with a Varian capillary column CP7502 (racemization)
or on a Chromapack CP-Chirasil-Dex CB column (DKR). (S)-1-
Phenylethanol was obtained by enzymatic kinetic resolution of the
racemate and purified by column chromatography. Commercially
3/7/24 69/85/94 Ͼ99/Ͼ99/Ͼ99
3/7/24 63/71/89
3/7/24 50/62/89
3/7/24 69/81/50
3/7/24 64/81/90
99/99/99
98/96/94
98/97/94
98/98/97
5b[b]
5b[b]
[a] 0.5 mmol Na2CO3. [b] 1.5 mmol Na2CO3. [c] tBuOK (1 )/THF
(5 mol-%). [d] By chiral GC.
It is well known that, even under dry reaction conditions,
the residual water in enzyme preparation always results in
some hydrolysis of the activated acyl donors (here isopro-
penyl acetate).[14,15] Whereas in conventional KR this usu- available (S)-1-phenylethanol (Aldrich, 97%) gives unreproducible
results. Likewise, potassium tert-butoxide, required for activation
of the racemization catalyst, should be freshly sublimated in order
to obtain reproducible results. Racemic 1-phenylethanol was ob-
tained from Fluka, tetraphenylcyclopentadienone and 1-(2-furyl)-
ethanol were obtained from Aldrich. CAL-B was purchased from
ally can be considered as a harmless side reaction, in DKR
the Ru catalyst present may be unstable towards the acid
formed. Enzymatic hydrolysis of the acylated alcohol prod-
ucts (R)-7 and (R)-9 may also take place to some extent,
especially at high conversions.
Due to the instability of ruthenium catalysts in the pres-
ence of acids, an effective racemization of (S)-6 and (S)-8
in DKR requires that the acetic acid released is neutralized
in situ. Accordingly, higher amounts of Na2CO3 were ob-
Novozymes. Pentabenzylcyclopentadiene was prepared by
a
method described in the literature.[10] CCDC-713844 (5b) and
-713845 (5c) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
served to afford somewhat higher conversions (Table 2, En- data_request/cif.
Eur. J. Org. Chem. 2009, 1317–1320
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1319

L. T. Kanerva, R. Leino et al.
SHORT COMMUNICATION
Dicarbonylchloro(pentabenzylcyclopentadienyl)ruthenium
(5b):
(12.5 or 5 mol-%), Ru-catalyst 5b or 1 (10 or 4 mol-%), CAL-B
(3 mg) and anhydrous Na2CO3 (0.5 or 1.5 mmol) under argon in a
Schlenk tube. The racemic substrate (0.201 mmol or 0.498 mmol)
in toluene (0.2 mL) was added with a syringe after 6 min; 4 min
later, isopropenyl acetate (0.30 mmol or 0.75 mmol) in toluene
(0.2 mL) was added to initialize the acylation. The reactions were
monitored by taking samples at intervals, filtering and analyzing
them by GC after derivatization with propionic anhydride in the
presence of pyridine and DMAP (1%). Dodecane or undecane was
used as a standard for quantitative analyses.
Complex 5b was prepared by two routes: Deprotonation of penta-
benzylcyclopentadiene with nBuLi followed by reaction with
[Ru(CO)3Cl2]2 provided low yields only (for details, see Supporting
Information). Much better results were obtained by a one-pot chlo-
rination of the corresponding hydride 5a, prepared in a manner
similar to the synthesis of dicarbonylchloro(pentaphenylcyclopen-
tadienyl)ruthenium (1). Thus, a mixture of C5Bn5H (310 mg,
0.6 mmol), Ru3(CO)12 (128 mg, 0.2 mol), decane (6 mL) and tolu-
ene (3 mL) were heated at 160 °C in a closed vessel for 72 h. The
vessel was cooled to room temp., opened, chloroform (1 mL) was
added and the heating continued at 150 °C for 2 h. After cooling to
room temp., the mixture was concentrated in vacuo and the residue
purified by column chromatography to provide 225 mg (53%) of 5b
Supporting Information (see footnote on the first page of this arti-
cle): Additional experimental details, NMR spectra and selected
crystallographic data for complexes 5b and 5c.
1
as a yellow microcrystalline solid. H NMR (600.13 MHz, CDCl3,
Acknowledgments
25 °C): δ = 3.57 (s, 10 H, CH2), 6.88 (ur, 10 H, m-H), 7.12–7.13
(m, 15 H, o- and p-H) ppm. 13C NMR (150.9 MHz, CDCl3, 25 °C):
δ = 30.72 (CH2), 105.26 (Cp ring), 126.72 (o-C), 128.47 (p-C),
128.57 (m-C), 137.93 (i-C), 197.10 (C=O) ppm. MS: exact mass
calcd. for C42H35102Ru35ClO2(+Na) 731.1267, found 731.1271; ex-
act mass calcd. for C42H35102Ru35ClO2(+K) 747.1006, found
747.0970; exact mass calcd. for C42H35102Ru35ClO2(+Na/CO)
703.1318, found 703.1310; exact mass calcd. for C42H35102Ru-
35ClO2(–Cl) 673.1681, found 673.1643.
Financial support from the Finnish Funding Agency for Technol-
ogy and Innovation (Technology Programme SYMBIO Project
#40168/07: Developing New Chemoenzymatic Methods and Bio-
catalysts) is gratefully acknowledged. The authors thank Markku
Reunanen for recording of mass spectra.
[1] P. M. Dinh, J. A. Howarth, A. R. Hudnott, J. M. J. Williams,
W. Harris, Tetrahedron Lett. 1996, 37, 7623–7626.
[2] a) A. L. E. Larsson, B. A. Persson, J.-E. Bäckvall, Angew.
Chem. Int. Ed. Engl. 1997, 36, 1211–1212; b) B. A. Persson,
A. L. E. Larsson, M. Le Ray, J.-E. Bäckvall, J. Am. Chem. Soc.
1999, 121, 1645–1650.
Dicarbonylchloro(tetraphenylbenzylcyclopentadienyl)ruthenium (5c):
nBuLi (0.21 mL of a 1.6  solution in hexane) was added to a solu-
tion of C5Ph4BnH (153 mg, 0.33 mmol) (for preparation, see Sup-
porting Information) in THF (2 mL) at –60 °C. The mixture was
stirred for 20 min followed by warming to room temp. Next, a solu-
tion of [Ru(CO)3Cl2]2 (102 mg, 0.2 mmol) in THF (1 mL) was
added. The mixture was stirred at room temp. for 2 h, concentrated,
and purified by column chromatography (hexane/DCM, 3:1Ǟ1:1)
to provide 42 mg (20%) of 5c as a yellow crystalline solid. 1H NMR
[3] For reviews, see: a) O. Pàmies, J.-E. Bäckvall, Chem. Rev. 2003,
103, 3247–3262; b) B. Martín-Matute, J.-E. Bäckvall, Curr.
Opin. Chem. Biol. 2007, 11, 226–232.
[4] Y. Ahn, S.-B. Ko, M.-J. Kim, J. Park, Coord. Chem. Rev. 2008,
252, 647–658.
[5] B. Martín-Matute, M. Edin, K. Bogár, F. B. Kaynak, J.-E.
Bäckvall, J. Am. Chem. Soc. 2005, 127, 8817–8825.
[6] a) J. H. Choi, Y. H. Kim, S. H. Nam, S. T. Shin, M.-J. Kim, J.
Park, Angew. Chem. Int. Ed. 2002, 41, 2373–2376; b) J. H.
Choi, Y. K. Choi, Y. H. Kim, E. S. Park, E. J. Kim, M.-J. Kim,
J. Park, J. Org. Chem. 2004, 69, 1972–1977.
3
(600.13 MHz, CDCl3, 25 °C): δ = 3.66 (s, 2 H, CH2), 6.60 (d, J =
3
7.7 Hz, 2 H, o-Bn) 7.00 (d, J = 7.7 Hz, 4 H, o-Ph), 7.04–7.10 (m,
7 H, m-Ph, m-Bn, p-Bn), 7.15 (ur, 2 H, p-Ph), 7.22–7.28 (m, 6 H,
m-Ph, p-Ph), 7.30 (d, 3J = 7.7 Hz, 4 H, o-Ph) ppm. 13C NMR
(150.9 MHz, CDCl3, 25 °C): δ = 31.25 (CH2), 102.30, 106.59,
117.21, 126.40 (p-Bn), 127.79 (o-Ph), 128.24 (m-Bn), 128.36 (m-Ph),
128.51 (p-Ph), 128.54 (p-Ph), 128.59 (o-Bn), 129.39, 129.75, 131.97
(m-Ph), 132.53 (o-Ph), 137.92, 196.98 (C=O) ppm. MS: exact mass
calcd. for C38H27102Ru35ClO2(+K) 691.0380, found 691.0369.
[7] a) C. P. Casey, S. W. Singer, D. R. Powell, P. K. Hayashi, M.
Kavan, J. Am. Chem. Soc. 2001, 123, 1090–1100; b) G.
Csjernvik, A. H. Eu, L. Fadini, B. Pugin, J.-E. Bäckvall, J. Org.
Chem. 2002, 67, 1657–1662.
[8] D. Mavrynsky, R. Sillanpää, R. Leino, Organometallics 2009,
28, 589–605.
Procedure for the Racemization of (S)-1-Phenylethanol: A solution
of tBuOK in THF (1 mL of 0.012 , 12 µmol, 3 mol-%) was added
to a solution of 1 or 5b (4 µmol, 1 mol-%) in toluene (2 mL). The
reaction mixture was stirred for 3 min, followed by addition of (S)-
1-phenylethanol (56 mg, 0.4 mmol) (Scheme S2, see Supporting In-
formation). Samples from the reaction mixture were acylated with
propionic anhydride in pyridine in the presence of DMAP.
Derivatization stops the racemization reaction and enhances the
resolution of peaks in the GC.
[9] R. M. Bullock, P. J. Fagan, E. M. Hauptman (DuPont), PCT
Int. Appl. 2001, WO 01/98241 A2.
[10] W.-M. Tsai, M. D. Rausch, R. D. Rogers, Organometallics
1996, 15, 2591–2594.
[11] S. Namorado, M. A. Antunes, L. F. Veiros, J. R. Ascenso,
M. T. Duarte, A. M. Martins, Organometallics 2008, 27, 4589–
4599.
[12] J. W. Chambers, A. J. Baskar, S. G. Bott, J. L. Atwood, M. D.
Rausch, Organometallics 1986, 5, 1635–1641.
[13] The molecular structure of 5c shows a disorder, in which the
Cl ligand and one of the two CO ligands have two orientations
in a 1:1 ratio. In Figure 5, only one orientation of the disor-
dered ligands is shown. For a more detailed illustration, see
Supporting Information.
Kinetic Resolution (KR) of rac-6 and rac-8: In a typical procedure,
the racemic substrate (0.2  or 0.5 ) in dry toluene (1 mL) and
the CAL-B preparation (3 mg) were added in a vial. The addition
of isopropenyl acetate (0.3  or 0.75 ) initialized the acylation.
The reactions were monitored by taking samples at intervals, filter-
ing and analyzing them by GC after derivatization with propionic
anhydride in the presence of pyridine and DMAP (1%).
[14] L. Veum, U. Hanefeld, Tetrahedron: Asymmetry 2004, 15,
3707–3709.
[15] L. Veum, L. T. Kanerva, P. J. Halling, T. Maschmeyer, U.
Hanefeld, Adv. Synth. Catal. 2005, 347, 1015–1021.
Received: December 16, 2008
Dynamic Kinetic Resolution (DKR) of rac-6 and rac-8: In a typical
procedure, dry toluene (0.6 mL) was added to a mixture of tBuOK
Published Online: February 3, 2009
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Eur. J. Org. Chem. 2009, 1317–1320